The invention relates generally to an exhaust treatment system and method of operating the exhaust treatment system.
Exhaust streams generated by the combustion of fossil fuels in, for example, furnaces, ovens, and engines, contain nitrogen oxides (NOx) that are undesirable pollutants. There is a growing need to have efficient and robust exhaust treatment systems to treat the NOx emissions.
In selective catalytic reduction (SCR) using hydrocarbons (HC), hydrocarbons serve as the reductants for NOx conversion. Hydrocarbons employed for HC-SCR include relatively small molecules like methane, ethane, ethylene, propane and propylene as well as longer linear hydrocarbons like hexane, octane, etc. or branched hydrocarbons like iso-octane. The injection of several types of hydrocarbons has been explored in some heavy-duty diesel engines to supplement the HC in the exhaust stream. From an infrastructure point of view, it would be advantageous to employ an on-board diesel fuel as the hydrocarbon source for HC-SCR.
The use of fuels, including gasoline or diesel fuel as SCR reductants, leads to a number of disadvantages when trying to clean up the exhaust gases. During the combustion process, the catalyst may get poisoned by some part of the exhaust gas, such as sulfur dioxide (SO2), or from the formation of base metal compounds from the components of a catalyst composition. The NOx absorption performance of the catalyst declines as the poisoning of the catalyst increases. Therefore, it is desirable to have an exhaust treatment system and method of operation that will help to mitigate poisoning and increase the catalyst performance.